Journal of Engineering Sciences, Assiut University, Vol. 39, No 3, pp.485 -496, May 2011

485

PERFORMANCE OF EXTERNAL REINFORCED CONCRETE BEAM-COLUMN JOINTS MADE OF HIGH STRENGTH

CONCRETE

Ramadan M. A.*, M. M. Ahmed**, Abdel-Mageed M. A.**, and Megahid A. A.*** * Eng.at Qune govnovate, ** Associate professor , Civil Eng. Department, Assiut University, *** Professor , Civil Eng. Department, Assiut University,

(Received January 27, 2011 Accepted April 16, 2011)

The integrity of beam-column joints in reinforced concrete (RC) frames, especially in a crucial zone, is essential for the satisfactory performance of the whole structure. Therefore, the behavior of external connections between beams and columns is a significant parameter affecting the performance of such R.C frames. An important influence in this regard is the concrete compressive strength, joint reinforcement and relative stiffness of beam and column in each connection, which is itself determined by considerations of geometry and reinforcement percentage. To contribute to a better understanding of the behavior of external beam-column joints made of high strength concrete a theoretical study based on the Non-linear finite element analysis is usually appropriate. ANSYS 10[17] software package was used to perform the analysis using the available Solid 65 3-D reinforced concrete element with reinforcing bars. 26 specimens were considered and analyzed. The main parameters were, the details, type and amount of joint reinforcement, the type and amount of beam and column main reinforcement in the joint, the level of axial column load and the concrete strength .The Obtained results were analyzed and discussed. The results were also compared with the obtained results using some available formulae found in the literature. Finally beneficial conclusions and some design recommendations were outlined and given.

KEYWORDS: joints, beam-column-connection, shear strength, high strength, concrete, joint reinforcement, R. C. frames.

1- INTRODUCTION

There is no doubt, that beam-column joints in R.C. ductile frames require special attention. Unless the designer possesses a good understanding of joint behavior and the reasons for possible unsatisfactory structural response an adequate design cannot be applied. Over the last 30 years, many theoretical and experimental research works have been carried out in several countries, which led to the identification of critical features of joint behavior. Recommendations for design, which are often empirical, were given in the different codes. In the recent years, there has been a rapid growth in the use of high strength concrete. However most of the previous works carried out so far for beam-column joints are for those made of normal strength concrete. A rational and

Ramadan M. A., M. M. Ahmed, Abdel-Mageed M. A., 486

simple design procedure for such connections especially when using high strength concrete, is still needed. The efficiency of using new non-conventional reinforcing pattern such as the use of fiber concrete or crossed inclined joint bars requires more researches.

Many parameters influence the strength and ductility of R.C connections including relative stiffness between beam and column, strength of concrete, type, quantity and detail of joint reinforcement and the column acting axial load. From the previous works, it can be shown that the strength of beamcolumn joint is limited by the following failure modes: joint shear failure of inadequate joints, column failure by forming plastic hinges in the column immediately above or below the beam in case of stiffer beams, beam flexural failure by forming plastic hinges (diagonal splitting failure, shear compression failure). The columns must remain essentially elastic throughout the load history to insure the lateral stability of the structure. At the same time, excessive joint shear deformation is irreversible and would cause permanent drift of the building [1-9].

Vollumn and Newman [3] in their study on external beam-column joints proposed an analytical model for the design and prediction of the strength of such connections. Tsonos et al [4] in their experimental investigation of external beam-column joints, found that inclined bars in the joint region improve their seismic resistance. Committee 352 [12] specifies maximum allowable joint shear stress for exterior joint of 0.96fcu. A lower limits of flexural strength ratio "MR" (the ratio of the sum of flexural capacity of columns to that of beam) in this case is confirmed to 1.4.

The present investigation aims to provide some information towards better understanding and satisfactory improving the strength and ductility for such connections, especially when using high strength concrete. Using a theoretical analysis through the ANSYS 10[17] software package, stresses, strains, crack pattern at different load levels, and the failure load, were predicted. The available Solid 65 3-D reinforced concrete element with reinforcing bars is used. The solid element is capable to cracking in tension and crushing in compression.

2- DETAILS OF CHOSEN EXTERNAL JOINTS AND

ARRANGEMENT OF ACTING LOADS 26 external R.C joints arranged in 5 series were analyzed. The description of various parameters considered in the study and the used program is summarized and given in table (1). The details of the joints are shown in Fig (1). The beam and the column for all specimens have the same cross section dimensions 25x50 cm. The main parameters taken into consideration are the concrete strength (Seri 1), the axial column load (Seri 2), the amount and type of extra horizontal top and bottom reinforcement at beam( Seri 4), the cross-bar reinforcement ratio at joint (Seri 5). The shear force was applied at the end of the beam and was incrementally increased up to failure, while, during each loading, the axial column load was kept constant at a certain value, which was less than 40 percent of the column ultimate axial load. The aim of subsequent loading was to trace the joint strength envelope.

PERFORMANCE OF EXTERNAL REINFORCED CONCRETE 487

Table (1) Details of the chosen external joints

Extra% cross Joint steel

Extra h.al

beam reinfh

Coln reinf c%

Beam main reinf b%

horial stirr.

of Joint

N/ fcubt

Column axial

Load (N) (KN)

fcu ( Mpa)

Joint No

Series No

-

-

1.3

0.67

18

0.32

800

20 J11

1 - 0.16 40 J12 - 0.11 60 J13 - 0.08 80 J14 - 0.064 100 J15 -

-

1.3

0.67

18 0 o.w.t

20 J21

2

- 0.08 200 J22 - 0.16 400 J23 - -

1.3

0.67

18 0 o.w.t

60 J24

- - 0.026 200 J25 - - 0.055 400 J26 -

-

1.3

0.67

18 0 o.w.t

100 J27

- 0.016 200 J28 - 0.026 400 J29 -

-

1.3

0.33

38

0.11

800

60

J31

3 - 1.0 0.11 J32 - 2

0.67 0.11 J33

- 3 0.11 J34 - 0.2

1.3

0.67

38 0.11

800

60 J35

4 - 0.4 0.11 J36 - 0.67 0.11 J37 - - 1.3 .67 58 0.11 800 60 J41

5 - 48 0.11 J42

0.2 - 1.3

0.67

38

0.11 800

60

J51 0.4 - 0.11 J52 0.67 - 0.11 J53

3- RESULTS AND ANALYSIS

The obtained failure load, peak strains, joint shear stress at failure and the mode of failure are given in table (2). To confirm the obtained ANSYS results, the failure loads for the joints were predicted using some theoretical approaches and formulae found in the code and literature (11). The estimated values are included in table (2) and are plotted against the corresponding obtained results for analyzed beams in Fig (2), where a good agreement can be noticed. However, the code formulae over estimated the failure load for those joints failed by joint shear.

Ramadan M. A., M. M. Ahmed, Abdel-Mageed M. A., 488

Fig (2) The obtained ANSYS results against the estimated failure load given in the available literature

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 Estimated Q-beam at failure KN[11]

AN

SYS

- O

btai

ned

Q-b

eam

at

failu

re K

N

PERFORMANCE OF EXTERNAL REINFORCED CONCRETE 489

Table (2) Some obtained and estimated results of the analyzed beams

Peak strain

At joint

Mode of failure

jf/

max,j Jo

int

Fai

lure

Sh

ear

jf

Mpa

Qbf Esti-

mated (KN)

Qbf at failure (KN)

Nc/ fcubt

Column Load Nc

(KN)

fcu (Mpa)

Joint No Se

ries

N

o

0.0017 3 0.25 0.95 - 50 0.32

800

20 J11

1 0.0018 3 0.27 1.41 - 80 0.16 40 J12 0.0025 3, 2 0.33 2.1 115 120 0.11 60 J13 -0.003 2 0.3 2.25 129 141 0.08 80 J14

-0.0028 2 0.28 2.33 132 152 0.064 100 J15 0.0015 3 0.13 0.5 - 27.1 0 o.w.t

20 J21

2

0.0016 3 0.17 0.62 - 34.5 0.08 200 J22 0.0016 3 0.22 0.79 - 42 0.16 400 J23 -0.0019 3 0.18 1.2 - 68.8 0 o.w.t

60 J24

-0.0018 3 023 1.5 - 86.4 0.026 200 J25 -0.0017 3 028 1.76 - 103 0.055 400 J26 0.0032 3 021 1.74 - 107 0 o.w.t

100 J27

0.0051 2 0.27 2.24 124 130 0.016 200 J28 0.005 2 0.29 2.3 132 144 0.026 400 J29

-0.0031 2 0.16 1.02 78 92.2 0.11

800

60

J31 3

-0.0027 2 0.41 2.62 139 152 0.11 J32 -0.0032 2 0.34 2.17 122 124.8 0.11 J33 -0.0034 2 0.37 2.25 122 127.3 0.11 J34 0.0041 2 0.38 2.5 136 142.9 0.11

800

60 J35

4 0.0037 1.2 0.46